N-Benzyloxycarbonyl-L-threonine amide (hereinafter referred to as Z-L-Thr-NH.sub.2) is useful as a starting material for the synthesis of monocyclic .beta.-lactam antibiotics and is disclosed in the specification of British Patent Laid-Open No. 2,071,650. According to this specification, Z-L-Thr-NH.sub.2 was extracted from the reaction mixture with ethyl acetate and then purified and isolated by using a solvent mixture of ethyl acetate and n-hexane.
In experiments conducted according to said process, the inventors have found that the filtration of the intended product was quite difficult and Z-L-Thr-NH.sub.2 was obtained in the form of an anhydrous, amorphous powder which was hygroscopic.
The difficulty of the filtration of the substance is a fatal defect in the mass production and the hygroscopicity of the obtained substance is undesirable for the storage and measurement thereof.